Every so often my screen kind of flashes and then I hear a distant explosion? Anyone know what it is? Is it happening on the map, or I was maybe thinking nukes outside of the game area?

Probably cars expolding, it keeps happening to me as well.

The different enemies and groups are roaming the world at the same time as you, and will get into fights if they encounter each other. Some of them have Fatman weapons, or carry mini-nukes like the Super Mutant Suiciders. So you'll occasionally see distant nuclear explosions from those fights. Gunfire can also set off car explosions, etc.

Far away Super Mutants that randomly fire Fat Mans off into the distance.

Cars, suicides, fat man, exploding veribirds

Yes - it is a loot opportunity.

While I was trying to clear the hospital filled with supermutants, some robots decided to get in on the action. A supermutant suicider ran into the middle and exploded. I tell you, those explosions are pretty dramatic seen through a scope!

What's point of vertibirds anyway? They seem to blow up whenever they come into contact with air

The vertibirds drop off BOS patrols and provide fire support. I have witnessed several encounters with BOS forces and the Vertibird sweeps in and pounds them with the minigun. Usually solves the issue though I did witness an encounter of BOS vs supermutants. The supermutants put in a hell of a fight took 3 vertibirds, 2 BOS knights, 4-5 othe BOS troops and at the end only one heavily damaged vertibird, and the almost dead knights were left, very good pickings after the fight was over that was a fun one to watch took about 4-5 minutes

Well in my game they blow up seconds after they show up

I was near a hut that suddenly exploded.....moonshine was involved I believe.

As to the OP, yeah you can sometimes follow the sound and head towards the weapons fire/explosions for some nice loot. My Red Rock base settlers are all wearing BOS scribe armor or Brotherhood fatigues.

lol I saw them clean up raider camps, so much loot and thank god for the strong back.

Actually that was me. Who said that 200 year old beans wouldn't be harmful...

I think you're talking about the radiation storms? Does the area you are in go green and smoggy too?

I was creeping up on one raider base when the Super Mutants next door kicked off with a Vertibird and blew it up... I see the flash and then heard the boom and thought nothing of it... until the damned cockpit hit me on the back of the head and pinged off into the distance.

I do love how Beth actually included the sound delay after the flash... gives it better immersion.

Bethesda did really good job on sound design on Fallout 4.

The way the distance gunshots echo in different ways in the city versus the countryside is really well done. A raider yelled at me in a parking garage, and there was a really nice and accurate echo and reverb that matched the space perfectly too.

The only thing bad about the sound design is the talking. Seriously.... you can hear someone who appears to be right next to you and in actual fact they can be three floors down and on the other side of the building. There just is no way to figure out where they are talking from from by just listening. Other games get it right... but a lot of the time in Fallout 4 the talking is just way too loud and clear (which sounds weird when talking about a game... but when you use every sense to locate an enemy, messed up audio cues ruin the immersion factor).

Take Destiny for example... the audio cues in that game are sublime... the game is lame but the gunplay and audio is bang on. You could close your eyes and just use audio to track enemies. In F4 though I was at the top of that flooded quarry that got drained... and I heard the enemy behind me when I was at the top... only he was at the back of a cave at the very bottom.

I love the sound of distant explosions & gun fire, makes me feel that the world I'm in is alive with activity.